The primary functions of the kidneys are to filter and excrete metabolic waste products. The kidneys help maintain homeostasis by regulating acid-base balance, blood pressure, and fluid volume. Renal function is assessed to determine renal sufficiency, renovascular disease, metabolic disorders, and other conditions. Assessment of renal perfusion is helpful in analyzing several renal diseases, for analyzing renal artery stenosis (RAS), for analyzing renal transplant dysfunction (e.g., chronic ischemic nephropathy, drug nephropathy), and for other purposes.
Since the kidney has different regions, compartmental models have been used to study the kidney. A compartmental analysis is a form of deterministic analysis that divides a physiological system into a number of interconnected compartments. A compartment may be an anatomical, physiological, chemical, or physical subdivision of a system. A compartmental model may be characterized by number of compartments, number of inputs, or number of outputs. In a deterministic model, analytical expressions are used to describe behavior. This compares to a stochastic model where behavior is determined by random processes that are described by probability functions. Since the kidneys have a single blood supply, the kidney may be studied using a single input, dual compartment model. The kidney may be represented as a combination of at least two homogeneous compartments, a vascular component and a tubular component.
Magnetic resonance imaging (MRI) provides highly detailed anatomical information. Dynamic contrast-enhanced (DCE) MRI of the kidney monitors the transit of contrast materials (e.g., gadolinium (Gd) chelates) through the intrarenal regions, the renal cortex, the medulla, and the collecting system of the kidneys. MRI using DCE may experience several stages. For example, at a first time, a bolus of contrast agent may arrive in the large vessels of the kidney. Cortical enhancement may then occur between approximately 20 and 30 seconds after the contrast agent is administered. This cortical enhancement may reflect the contrast within the renal vasculature. Medullary enhancement may then occur approximately 60 seconds after cortical enhancement. The medullary enhancement may be dominated by contrast in renal tubules. Enhancement in the collecting system of the kidney may then occur starting around 180 seconds after medullary enhancement. By analyzing the enhancement at various time points after administration of contrast agent, clinically relevant parameters including renal blood flow (RBF), glomerular filtration rate (GFR), cortical blood volume, and medullary blood volume may be measured. However, acquiring sufficient signals to perform quantization that is sufficient to support meaningful functional analysis requires a combination of spatial resolution and temporal resolution that was not conventionally available.
Conventionally, different methods have been used to quantify renal perfusion using information acquired by MRI. These methods included the upslope method, semi-quantitative parametric methods, de-convolution methods, and various compartmental models. Unfortunately, the temporal resolution provided by conventional MRI systems may not have been sufficient to support functional examinations. Additionally, applying conventional under-sampling to improve temporal resolution may have negatively impacted spatial resolution to the point where functional examinations were difficult, if even possible at all, to achieve.
Quantitative evaluation of renal functions including perfusion and filtration is employed for diagnosing and monitoring vascular diseases, hypertension, obesity, diabetes, renal transplantation, urinary tract obstruction, and other processes. While conventional systems have provided some insights into these conditions and other conditions, conventional systems have been limited by imaging technology that could not provide dynamic acquisitions with adequate spatial resolution and temporal resolution to provide dynamic quantitative data from functional examinations.
Conventional studies have typically employed T1-weighted, gradient recalled echo (GRE) sequences. T1 refers to spin-lattice relaxation, T2 refers to spin-spin relaxation. Three-dimensional (3D) acquisitions may provide continuous whole-kidney coverage to assess whole-kidney function, but have been limited by longer acquisition times. 3D T1 mapping within one breath-hold has typically been challenging. Thus, two-dimensional (2D) images have typically been acquired with higher temporal and spatial resolution. However, the 2D image approach may have been limited to a single representative slice or selected slices, which precluded whole kidney functional analysis. Achieving higher temporal and spatial resolution facilitates achieving greater precision in estimating renal perfusion and filtration rates.
Kinetic modeling involves converting an MRI signal into a gadolinium (Gd) concentration. This conversion has been challenging because MR signal intensity varies with contrast agent concentration, pulse sequence parameters, pre-contrast relaxation times, blood flow velocity, and other factors. Additionally, the relationship between signal and concentration is non-linear. Conventional spatial and temporal resolution may have been insufficient to provide adequate signal for meaningful functional analysis involving kinetic modeling.